Presently most LWIR HgCdTe photodiodes are double layer heterojunction (DLHJ) structures which are fabricated by etching a plurality of mesas to isolate the individual photodiodes of an array of photodiodes. However, the resulting non-planar surface which results from the mesa etch has been found to be difficult to passivate. Such passivation is generally applied as a layer over the surface of the mesa structures in order to control surface states. The non-planar surface is also less stable than a planar surface due to non-uniform surface coverage and relatively poor adhesion of the passivation layer to the underlying material. Furthermore, in a mesa structure the active narrow bandgap/wide bandgap p-n junction intersects the surface of the device by being exposed along the mesa walls. Thus, the junction is exposed to enhanced thermally generated pair (g-r) noise associated with surface states and to flat-band voltage shifts which result from uncontrolled charges in the overlying passivation layer.
Another problem associated with conventional mesa-type structures is that the array of photodiodes must be removed from a growth chamber in order to etch the individual mesas. Thus, the fabrication of these conventional devices is more complex and costly. Also, inasmuch as the device must be removed from the controlled atmosphere of the growth chamber in order to etch the mesas, there is increased susceptibility to surface contamination and other undesirable factors which may influence the noise and spectral response of the resulting array.
In a journal article "Development of HgCdTe LWIR Heterojunction Mosaics", Proc. IRIS Detector, 1986, Vol II, Wang et al. discuss at pages 255, 256 and show in FIG. 8 a p-on-n LWIR planar detector grown by LPE which has individual diodes delineated by ion-implantation. Wang does not teach a thermally driven delineation process nor a process amenable to in-situ execution within a growth chamber. Furthermore, ion-implantation may induce implant-damage related noise effects.
It is thus one object of the invention to provide for an array of LWIR HgCdTe photodiodes which has a planar surface as opposed to a non-planar surface, the individual photodiodes being delineated in-situ within a growth chamber.
It is another object of the invention to provide an LWIR HgCdTe array of photodiodes which has improved surface coverage and adhesion to an overlying passivation layer or layers, the passivation layer also being deposited in-situ.
It is another object of the invention to provide an LWIR HgCdTe array of photodiodes wherein the active narrow bandgap/wide bandgap p-n heterojunction is not exposed at the surface of the array thereby improving the noise characteristics of the photodiodes.
It is still one further object of the invention to provide a method of fabricating an array of photodiodes which provides for isolating the individual photodiodes of the array in-situ within a reactor following double-layer deposition.
It is still one further object of the invention to provide a planar double-layer heterojunction photodiode structure which comprises four distinct junctions, two heterotype and two isotype, certain of these junctions defining an isolation junction region which is formed in a collector layer and possibly a base layer by a thermally driven method of selective type-conversion, the isolation junction region physically and electrically isolating individual photodiodes of the array one from another.